The amphiphilicity and surface activity properties of a substance, such as of a drug, correlate to its adsorption in the gastro-intestinal tract, to its distribution in the tissues and especially to its blood-brain barrier (BBB) permeability, liver metabolism and urinary excretion, that is to its so-called ADME properties. Amphiphilicity and detergent properties have conventionally been determined by measuring the effect of the substance on the surface tension of water. Another conventionally used method for estimating ADME properties is to determine the partition coefficient (log P) of the substance in octanol/water.
When an amphiphilic substance is added to an aqueous solution, the substance partitions into the air/water interface, causing a decrease in the surface tension (increase in the surface pressure). Surface tension can be measured for example by measuring the force applied to a probe in the air/water interface. Such a probe can be in the form of a thin platinum plate such as a Wilhelmy plate, which is placed in the air/water interface. An alternative construction for the probe is in the form of a small diameter metal wire.
A change in surface tension is evidenced as a change in the amount of liquid adhered to the probe. When the surface tension of the liquid decreases, the amount adhered to the probe decreases linearly, and vice versa. The liquid adhered exerts a vertical force on the probe, which can be detected using a microbalance.
Generally, surface tension measurement techniques require large reagent volumes and long experimental times. The huge increase in screening of new drug candidates requires, however, fast and reliable techniques applicable over a wide surface tension range. Additionally, for economic reasons, miniaturization of the reagent volumes is especially desirable for expensive substances, such as drug candidates.